1. Field of the Invention
The present invention relates to a fingerprint sensor using a piezoelectric membrane. More particularly, the present invention relates to a fingerprint sensor for sensing a fingerprint by converting pressure applied to a piezoelectric membrane into an electrical signal.
2. Description of the Related Art
Socioeconomic developments have led to off-line direct transactions and cash transactions in the purchase of and payment for goods. To facilitate such transactions, a new form of payment, i.e., using credit cards or on-line e-money, has emerged. As a result, security of one's private information has become a critical issue, and an individual fingerprint sensor is needed to protect an individual's information. In addition, a technique for sensing and deciphering a fingerprint in-situ, where private information is needed, is required.
FIG. 1 shows a perspective view of a conventional fingerprint sensor. Referring to FIG. 1, if a piezoelectric thick film 11 is pressed by a finger, a current density on a contact portion 9 between a fingerprint 12 and the piezoelectric thick film 11 is changed by pressure 13 due to the fingerprint 12.
The current density is read to an output line 15 through sensing devices 14 to sense the fingerprint 12. However, since information is read using a thick film and not a membrane, it is difficult to precisely measure information and to manufacture the fingerprint sensor as a one-chip type. Accordingly, there is a problem in that it is difficult for an individual to carry the fingerprint sensor, and therefore simple applications to portable systems are difficult.
FIG. 2 shows a cross-sectional view explaining the operation of a conventional fingerprint sensor. This type of conventional fingerprint sensor uses the principle of a capacitive CMOS sensor. Capacitance between sensor surfaces 22 on a substrate 21 and a fingerprint 23 is measured, converted into an electrical signal, and realized as a digital image.
However, this way is very sensitive to temperature and moisture. Additionally, there are problems in that additional circuitry and materials are necessary because of static electricity and it is difficult to draw regular results. Moreover, the quantity of charge readable from the fingerprint and sensor surfaces when operating the fingerprint sensor is small and thus must be separately stored by repeatedly toggling a switch on and off.